The present invention relates to light polarizers or more particularly to light polarizers having increased hydrolytic stability.
U.S. Pat. No. 4,025,688 teaches the manufacture of light polarizers by providing a film of polyvinyl alcohol with a dichroic stain such as iodine. The polarizer may optionally be supported by a layer of cellulose acetate butyrate bonded to the polarizer through a hydrophilic tie coat.
A problem with this type of polarizer is it demonstrates poor hydrolytic stability. That is, its polarizing efficiency, or ability to polarize light, decreases over time as heat and humidity increases. This tendency is disadvantageous for such uses as in liquid crystal displays especially for automobile instruments which must function in a wide range of heat and humidity conditions.
Czechoslovakian Pat. No. 122,408 discloses a process for stabilizing polarizing film by exposure to radiation at a wavelength of less than 400 nm for three hours.
Russian Pat. No. 124,116 stretches a polyvinyl alcohol film, heats it and then places the film into an iodized solution containing hydrobromic acid and hydriodic acid with subsequent washing and drying. There is no subsequent heating.
It has also been a problem in the art to produce a clear, thermoplastic film which is tough, chemically resistant and which is useful for optically critical purposes such as the production of polarizers. Biaxially oriented polyester film, while it is clear and tough, has an optic axis plane angle which varies from point to point across the web. This variation of orientation of the optic axis plane precludes the use of biaxially oriented polyester film for most such optically critical applications.
In the production of polarizing sheets, a film of a substrate is laminated to a film of polyvinyl alcohol. This laminate is then passed through an aqueous staining bath containing iodine and one or more iodine salts to tint the polyvinyl alcohol film. Stabilizing treatments may then be employed.
The present invention provides an improved hydrolytically stabilized polarizer with a tough, optically critical polyester substrate. Such improved substrate possesses a very low extinction angle, thus making it highly useful for polarizing applications, and at the same time demonstrates increased adhesion to the polyvinyl alcohol film to which it is laminated. It also demonstrates improved thermal dimensional stability, an improved moisture barrier and reduced water and solvent content.